By David Brunnstrom
WASHINGTON (Reuters) - Satellite images taken between January and this month show a North Korean nuclear reactor that can yield material for atomic bombs may be operating again at low power or intermittently, U.S. experts said on Wednesday.
A report from David Albright and Serena Kelleher-Vergantini at Washington's Institute for Science and International Security said the imagery also suggested that a centrifuge plant at the Yongbyon nuclear complex had been operated and that North Korea may be preparing to conduct renovations at this plant.
The ISIS think tank said last year that satellite imagery from late August and late September indicated the Yongbyon reactor may have been partially or completely shut down.
The latest ISIS analysis comes at a time of rising concern about North Korea's nuclear and missile capabilities.
The experts based their latest assessment on observation of melting snow patterns on the reactor and turbine buildings at Yongbyon, indicating that the insides of the buildings may have been hot. They also pointed to signs of warm water being discharged from the reactor.
North Korea has a uranium enrichment facility at Yongbyon and the reactor has previously been used for plutonium production. Both materials can be used to make atomic bombs.
In February, Albright was among experts at the U.S.-Korea Institute who presented three scenarios for North Korea's nuclear capability, predicting its stockpile of weapons could grow to 20, 50 or 100 within five years.
Last week, the Wall Street Journal newspaper reported that Chinese nuclear experts had warned that North Korea may already have 20 nuclear warheads and the capability to produce enough weapons-grade uranium to double its arsenal by next year.
These estimates, relayed to U.S. nuclear specialists, exceeded most previous U.S. forecasts, which ranged from 10 to 16 bombs currently, the report said.
Early this month, U.S. Admiral William Gortney, commander of the U.S. Northern Command and North American Aerospace Defense Command, said the U.S. military believes North Korea has the ability to miniaturize a warhead and mount it on a ballistic missile, although there had been no tests.
North Korea is under an array of international sanctions for nuclear bomb and ballistic missile tests. It has conducted three nuclear detonations, the most recent in February 2013.